An amplifier operating in the high-frequency band of 1 GHz or more has recently been under active development. Such an amplifier is used in e.g. radio communication equipment, mobile communication base stations, and radar devices. This amplifier is required to amplify and transmit signals in the high-frequency band at high power-added efficiency.
In a microwave semiconductor amplifying element, high efficiency operation called class-F mode can be achieved when the load impedance for the second harmonic seen from the output electrode end of the semiconductor amplifying element is made near-short, and the load impedance for the third harmonic is made near-open. In a known technique, an open-stub that is a tip-open transmission line of ⅛ wavelength of the fundamental is connected to the output electrode end of the semiconductor amplifying element so that the load impedance for the second harmonic seen from the output electrode end of the semiconductor amplifying element is made near-short. An open-stub that is a tip-open transmission line of 1/12 wavelength of the fundamental is connected to the output electrode end of the semiconductor amplifying element through a transmission line of ⅛ wavelength of the fundamental so that the load impedance for the third harmonic is made near-open.
Furthermore, in a microwave semiconductor amplifying element, high efficiency operation called inverse class-F mode can be achieved when the load impedance for the second harmonic seen from the output electrode end of the semiconductor amplifying element is made near-open, and the load impedance for the third harmonic is made near-short. In a known technique, the impedance for the second harmonic is controlled without affecting the fundamental by a harmonic impedance adjustment line so that the load impedance for the second harmonic is made near-open.